Vital signs fiber optic sensor systems and methods

ABSTRACT

An intensity-based, micro-bending optical fiber sensor is disclosed herein, which is configured to acquire clean, stable, and reliable vital sign signals. Related systems and methods for vital sign monitoring are also provided herein. The sensor of various embodiments includes a multi-mode optical fiber, an LED light source, an LED driver, a receiver, and a single layer deformer structure. In various embodiments, the optical fiber and single layer deformer structure of the sensor are selected to meet specific parameters necessary to achieve a level of reliability and sensitivity needed to successfully monitor vital signs. In some embodiments, a specific sizing relationship exists between the optical fiber and the single layer deformer structure. In some embodiments, the sensor is configured to acquire ballistocardiograph waveforms.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/872,040, filed on Sep. 30, 2015, which claims the benefitunder 35 U.S.C. §119(e) of U.S. Provisional Application 62/057,237,filed Sep. 30, 2014, which is herein incorporated by reference in itsentirety.

TECHNICAL FIELD

This application generally relates to the field of fiber optics, andparticularly, to optical fiber sensors and methods for monitoringphysiological parameters of a patient.

BACKGROUND

With the advent of internet-connected devices and the digital healthindustry, health and wellness monitoring has become an area of growingfocus. Monitoring vital signs such as heart rate, ballistocardiogramsignals, and breathing rate is desirable both inside and outsidehealthcare facilities. Within healthcare settings, vital sign trackingcan be essential for: ensuring patient safety when a healthcare provideris not present at a bedside, diagnosing medical conditions, monitoring apatient's progress, and planning a patient's care. Outside of healthcaresettings, tracking vital signs and posture enables individuals toquantify and conceptualize their health status, thereby helpingindividuals remain mindful of their health and wellness needs, visualizeprogress, and maintain the motivation needed to achieve health andfitness goals.

Current vital sign trackers in the consumer market are fairly intrusive,for example, current heart rate monitors often require an individual tostrap the monitor around the individual's chest. Many vital signtrackers include just one type of sensor configured to detect one typeof vital sign, such as, for example, heart rate. Additionally, manyvital sign monitors in the consumer market are not very accurate. In thehealthcare setting, much more accurate devices are available, but theyare often very large devices positionable at a patient's bedside,requiring a connection to an electrical outlet and leads attached to thepatient. Attachment to these bedside devices can cause anxiety inpatients, and the devices are expensive, not portable, and prone toelectromagnetic interference (EMI).

Optical fiber sensors have gained increased attention in the researchsetting as an alternative to existing vital sign monitors. Optical fibersensors are chemically inert and resistant to EMI. Moreover, they can beportable and integrated into fixtures, such as mattress pads andcushions. Fixture-integrated devices have numerous advantages overbedside appliances and wearable instruments. For example,fixture-integrated devices allow for a reduction in loose connectingwires or wireless data transmitters between sensors, electronics, andpower supplies. This reduction may lead to increased reliability, dataquality, and security.

However, optical fiber sensors developed to date have not proven to besuitable alternatives to conventional monitoring systems. For example,in “Optical Fibre Sensors Embedded into Medical Textiles for HealthcareMonitoring,” IEEE Sensor J. 8 (7), 1215-1222, 2008, Grillet el atproposed integrating a single mode macro-bending fiber sensor into abelt to measure respiratory rate. A macro-bending sensor typicallyexperiences significant light loss due to macroscopic deviations in thefiber's axis from a straight line, resulting in low sensitivity. Such asensor would be unlikely to detect the subtle movements of the chestwall needed to accurately measure heart rate or ballistocardiogramsignals.

In an effort to improve sensitivity, others have proposed alternativeapproaches for fiber optic sensors. For example, in U.S. Pat. No.6,498,652, Varshneya et al. disclosed a fiber optic monitor thatutilizes optical phase interferometry to monitor a patient's vitalsigns. Optical phase interferometry has several limitations. Forexample, while Fabry-Perot interferometric sensors and Mach-Zehnderinterferometric sensors are sensitive to mechanical vibrations of thebody, they are also highly sensitive to mechanical vibrations externalto the body, as well as temperature, acoustic waves, magnetic fields,and other environmental noise. Thus, without proper equipment,interferometer sensors are not suitable for monitoring vital signs dueto unreliable performance caused by signal fading and inaccuraciesresulting from environmental noise-induced phase change. The equipmentneeded to filter out the environmental noise includes an expensive phasemodulator and coherent optical sources, which add significant cost andcomplexity and make such sensors impractical for widespread commercialadoption. Other proposed designs have also struggled to balancesensitivity, accuracy, and cost.

Moreover, most fiber optic vital sign sensors being developed arelimited to detecting heart rate, breathing rate, and/or macro-movementsindicative of changes in body position. A major limitation of many ofthese sensors is the inability to obtain the highly sensitiveballistocardiography (BCG) waveforms. BCG is a technique used to recordvibrations of the body resulting from mechanical activity of the heart.In particular, BCG measures mass movements of the heart and circulatingblood generated by forces associated with heart contractions during thecardiac cycle. Historically, BCG waveforms were acquired using anextremely large, suspended table configured to support a patient lyingthereon; such a suspended table was heavy, non-portable, and requiredsubstantial mechanical maintenance. Due to the cumbersome systemrequired, BCG did not get much attention or use during much of thetwentieth century; however, reliable BCG waveforms can providesignificant insights into a patient's cardiac health. In addition torevealing a patient's heartbeat unobtrusively, in real-time, BCGwaveforms are useful in determining heart rate variability, which is anindicator of stress on a body. Moreover, comparison of BCG and EEGwaveforms, in particular, detection of the timing between the R peak ofthe EEG waveform and the J peak of the BCG waveform, revealsbeat-to-beat blood pressure changes. Additionally, as described, forexample, in E. Pinheiro et al., “Theory and Developments in anUnobtrusive Cardiovascular System Representation: Ballistocardiography,”The Open Biomedical Engineering Journal, 2010, 4, pp. 201-216, thecontents of which is herein incorporated by reference in its entirety,features of BCG waveforms have been found to correlate to, and suggestthe presence of, a number of maladies. For example, abnormal BCGwaveforms are obtained in individuals having angina pectoris,asymptomatic coronary artery disease, acute myocardial infarction,hypertension, coarctation of the aorta, and mitral stenosis, to name afew.

Despite the clinical value of monitoring BCG, it is not conventionallymonitored in a healthcare setting, due to a lack of a suitable detectionsystem. Detecting BCG waveforms requires a level of sensitivity andprecision that current sensor designs are lacking. Therefore, a needexists for a physiological parameter monitoring device capable ofreliably detecting BCG waveforms. A need also exists for a method ofdetecting vital signs, including BCG waveforms, which overcomes thelimitations of existing methods. Thus, there is a need for new anduseful optical fiber vital sign sensors and related methods of use.

SUMMARY

The present disclosure provides new and useful optical fiber sensors andrelated systems and methods for monitoring BCG waveforms and other vitalsigns. Various embodiments provided herein overcome one or more of theshortcomings of previously designed fiber optic vital sign monitoringsystems.

Various aspects of the disclosure are directed to an optical fiber vitalsigns sensor and related methods of vital signs detection. The vitalsigns sensor of various embodiments includes a single layer grid (e.g.,mesh) structure and a multimode optical fiber connected to an LED lightsource. The sensor of various embodiments is configured to achieve highsensitivity and low cost for the monitoring of heartbeat dynamics,breathing patterns, and body movements. The heartbeat dynamics,breathing patterns, and body movement of a patient's body cause micro-or macro-movements that exert forces onto the grid structure. Inresponse, the single layer grid structure applies a continuous force orpressure on the multimode optical fiber. The heartbeat dynamics,breathing patterns, and body movement each thereby exerts a force ontothe sensor, which causes micro-bending in the optical fiber of thesensor and thereby modulates the intensity of light transmitted in themultimode optical fiber. By monitoring the optical intensity changesdirectly coming out of the multimode optical fiber, the system derivesthe heartbeat dynamics, breathing pattern, and/or other body movementsignals.

Because the main sensor structure is composed of a multimode opticalfiber, one layer of a grid structure, one LED light source, and oneoptical signal receiver, the sensor of various embodiments achieves highsensitivity with very low cost. In particular, with these componentsunchanging across various embodiments, the sensor size and shape can bechanged to any size and shape with almost the same cost. In variousembodiments of the disclosure, an LED light source and optical signalreceiver are used to detect the optical intensity change. No additionaloptical coupler or optical interference structure is needed todemodulate the signal. Accordingly, the system cost is reduceddramatically compared to interferometry systems. In addition to costsavings, the use of minimum structures achieves high sensitivity. Thehigh sensitivity allows the sensor of various embodiments to detect andprovide detailed ballistocardiography waveforms in addition to othervital signs.

One aspect of the disclosure is directed to a sensor for detecting aphysiological parameter. The sensor of various embodiments includes amulti-mode optical fiber, an LED light source, an LED driver, areceiver, and a deformer structure. In some embodiments, the multi-modeoptical fiber includes an inner core, a cladding layer, and an outercoating, and in the optical fiber, a core diameter is greater than 50%of a cladding diameter. The LED light source is coupled to a first endof the optical fiber and emits light into the first end of the opticalfiber. The LED driver is electrically coupled to the LED light sourceand configured to regulate a power level of the LED light source toregulate an initial intensity of light emitted into the optical fiber.The receiver is coupled to a second end of the optical fiber andconfigured to sense changes in an intensity of light traveling throughthe optical fiber. The deformer structure consists of a single meshlayer formed of mesh having openings disposed therein. In someembodiments, a surface area of the openings is between 30% and 60% of atotal surface area of the mesh layer. The optical fiber of variousembodiments is arranged in a plane in contact with a surface of thedeformer structure such that an application of force onto the sensorresults in a first portion of the optical fiber bending into an openingof the mesh layer and a second portion of the optical fiber flexingagainst the mesh. In various embodiments, such deformation of theoptical fiber results in light loss through the cladding and therebymodulates the intensity of light reaching the receiver.

In some embodiments, this simplified optical fiber sensor is configuredto detect a ballistocardiogram of a patient. The optical fiber sensormay also be used to detect a heart rate, breathing rate, ormacro-movements of a patient.

As used herein, in various embodiments, the total diameter of theoptical fiber consists of the diameter across the inner core, thecladding layer, and the outer coating.

The mesh layer of some embodiments is formed of interwoven fibers. Insome embodiments, a diameter of each interwoven fiber is within 25% ofthe total diameter of the optical fiber. For example, in someembodiments, a diameter of each interwoven fiber is 75% to 125% of thetotal diameter of the optical fiber. In some embodiments, the interwovenfibers comprise a polymeric fabric. The mesh layer of some embodimentsis configured to uniformly distribute an applied force on the opticalfiber. In some embodiments, the opening of the mesh layer is 100% to300% of the total diameter of the optical fiber. In some embodiments,the opening of the mesh layer is 130% to 170% of the total diameter ofthe optical fiber.

The optical fiber of some embodiments is arranged in the plane such thata bending diameter of the optical fiber is greater than 1.5 cm. In someembodiments, the optical fiber is at least 10 meters long.

In some embodiments, the LED light source is a low power LED with a 1310nm or 850 nm central wavelength and 165 nm Full width at half maximum(FWHM). The optical fiber of some embodiments is coupled to the LEDlight source and the receiver via direct optical fiber connectorswithout the need for a separate lead fiber. In some embodiments, theoptical fiber has a numerical aperture less than or equal to 0.29.

In some embodiments, the sensor also includes a flexible outer coverenclosing the optical fiber and deformer structure. The outer cover ofsome embodiments is formed of silicone.

Another aspect of the disclosure is directed to a method of detecting aphysiological parameter. The method of various embodiments includespositioning a sensor under a body, wherein the sensor includes: amulti-mode optical fiber formed of an inner core, a cladding layer, andan outer coating, wherein a core diameter of the optical fiber isgreater than 50% of a cladding diameter; an LED light source coupled toa first end of the optical fiber; an LED driver electrically coupled tothe LED light source and configured to regulate a power level of the LEDlight source; a receiver coupled to a second end of the optical fiber;and a deformer structure. The deformer structure consists of a singlemesh layer having openings disposed therein, the openings having asurface area between 30% and 60% of a total surface area of the meshlayer. The optical fiber is arranged in a plane in contact with asurface of the deformer structure. The method of various embodimentsfurther includes: detecting, by the receiver, a change in an intensityof light traveling through the optical fiber, wherein the change inlight intensity corresponds to fiber deformation caused by a movement ofthe body, and determining a physiological parameter from the change inlight intensity.

The movement of the body may be a macro-movement, such as a change inthe body's position, or the movement may be a micro-movement, such as amovement caused by a contraction of the heart, the acceleration of bloodthrough the blood vessels, or the inspiration or exhalation of a breathby the body. In some embodiments, the physiological parameter that isdetermined by the method is or includes a ballistocardiography (BCG)waveform. In some such embodiments, determining the physiologicalparameter comprises: determining a BCG waveform of the body, determiningan electrocardiogram (EEG) waveform of the body, and calculating a timebetween an R peak of the EEG waveform and a J peak of the BCG waveformto determine beat-to-beat blood pressure changes.

In some embodiments, determining the physiological parameter includes:recording the signal detected at the receiver; converting the signal toa digital waveform; filtering out breathing and body movement waveformsfrom the digital waveform to extract a heartbeat waveform; identifyingheartbeat peak values from the heartbeat waveform by separating theheartbeat waveform into a first channel for time domain analysis andinto a second domain for frequency domain analysis; and applying a FastFourier transform (FFT) in the frequency domain to obtain the heartbeatrate value.

The above-mentioned features, as well as other features, aspects, andadvantages of the present technology will now be described in connectionwith various embodiments of the invention, in reference to theaccompanying drawings. The illustrated embodiments, however, are merelyexamples and are not intended to limit the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of an intensity-based fiber opticsensor in the prior art.

FIGS. 2A and 2B illustrate schematic cross-sectional views of anintensity-based fiber optic sensor in the prior art.

FIG. 3 illustrates a schematic diagram of one embodiment anintensity-based fiber optic sensor in accordance with the teachings ofthe present disclosure.

FIG. 4A illustrates a schematic cross-sectional view of one embodimentof an intensity-based fiber optic sensor in accordance with theteachings of the present disclosure.

FIG. 4B illustrates a schematic top view of one embodiment of a meshdeformer structure, such as the mesh deformer structure found in theintensity-based fiber optic embodiment of FIG. 4A.

FIG. 5 illustrates a schematic cross-sectional view of one embodiment ofa multi-mode optical fiber in accordance with the teachings of thepresent disclosure.

FIG. 6 illustrates a block diagram of one embodiment of anintensity-based fiber optic sensor system in accordance with theteachings of the present disclosure.

FIGS. 7A-7D illustrate one example of a raw digital signal, a combinedheartbeat and breathing waveform, a heartbeat waveform, and a breathingwaveform, respectively acquired from an embodiment of theintensity-based fiber optic sensor system of the present disclosure.

FIG. 8 is a flow chart illustrating of one embodiment of a method forfiltering and analyzing data acquired by an intensity-based fiber opticsensor system, in accordance with the teachings of the presentdisclosure.

FIGS. 9A and 9B are flow charts illustrating one embodiment of a methodof detecting a physiological parameter with an intensity-based fiberoptic sensor system, in accordance with the teachings of the presentdisclosure.

FIGS. 10A and 10B illustrate ballistocardiograph waveforms acquired,respectively, by a prior art optical fiber sensor system and an opticalfiber sensor system of the present disclosure.

DETAILED DESCRIPTION

The provided figures and the following description of certainembodiments of the invention are not intended to limit the invention tothese embodiments, but rather, are provided to enable any person skilledin the art to make and use this invention. New optical fiber sensors andrelated methods of using the optical fiber sensors are disclosed herein.In particular, embodiments utilizing the optical fiber sensors for vitalsign monitoring are disclosed.

Optical fibers can be used as sensors to measure strain, temperature,pressure, and other quantities by modifying a fiber so that the quantityto be measured modulates the intensity, phase, polarization, wavelength,or transit time of light in the fiber. There are four categories offiber optic sensors: intensity-based fiber optic sensors, spectrallybased fiber-optic sensors, interferometric fiber-optic sensors, andmultiplexed and distributed optical fiber sensors. Two of these sensortypes: interferometric fiber-optic sensors and intensity-based fiberoptic sensors have shown some promise in detecting vital signs and havebeen experimented with for such purposes in the prior art.

As described, for example, in U.S. Pat. No. 6,498,652 by Varshneya etal., the disclosure of which is herein incorporated by reference in itsentirety, an optical phase interferometry fiber optic sensor can achievehigh sensitivity. However, an important challenge lies in the need todifferentiate environmental perturbations such as temperature, strain,pressure, etc. from the desired signal. In principle, all environmentalperturbations could be converted to optical signals by applyingappropriate transducing mechanisms. However, sometimes multiple effectscan contribute simultaneously and modify the light in the fiber in asimilar manner. For example, changes in temperature, strain, pressure,or any mechanical perturbation could all impact the light in the fiberby changing fiber lengths and refractive indices such that it isdifficult to differentiate one perturbation from another. To overcomethis obstacle, very expensive, complex, high-maintenance equipment isneeded, such as an expensive phase modulator and coherent opticalsources. Accordingly, the high cost, complexity, and need for continuedmaintenance have limited the commercial use of such sensors for healthand wellness monitoring.

Intensity-based fiber optic sensors are far simpler sensors, capable offunctioning with a simple light source (e.g., a low power LED) and asimple detector without the need for phase modulators. However, it hasproved challenging for those in the field of optical fiber sensing todevelop a simple, low cost intensity-based fiber optic sensor that alsohas sufficiently high sensitivity to make this form of sensor a viable,practical option. Many types of optical intensity sensors have beendeveloped in an effort to create one that has sufficient sensitivity.One type of intensity-based fiber optic sensor, in particular, has shownsome promise: the microbending optical fiber sensor. Microbendingoptical fiber sensors rely on microbending of the fiber, resulting, forexample, from an externally applied force or pressure, to induce lightintensity modulation. Intensity modulation induced by microbending inmultimode fibers is considered as a transduction mechanism for detectingenvironmental changes such as pressure, temperature, acceleration, andmagnetic and electric fields. Microbending of the fiber locally resultsin intensity modulation of the light propagating through an opticalfiber. Microbending has been studied since the 1970s. However, there hasyet to be industry standard specifications or test methods, almost 40years later. That is because there are so many parameters that affectmicrobending fiber optic performance, including optical fiber length,reflective index, and optical core and cladding diameter, and deformerstiffness, material, periodicity, diameter, etc. Moreover, as describedin detail below, past researchers have failed to develop a microbendingoptical fiber sensor structure that is both sensitive and reliableenough to consistently and accurately detect BCG waveforms outside ofthe laboratory.

Microbending Optical Fiber Sensor

FIG. 1 illustrates a microbending optical fiber sensor in the prior art.The sensor 100 includes a dual deformer structure comprising a top meshlayer 102 and a bottom mesh layer 104, which together sandwich theoptical fiber 106. A first end of the optical fiber 106 is connected toa light source 108, and a second end of the optical fiber 106 isconnected to a signal receiver 110. The signal receiver is furthercoupled to a signal amplifier 112 and a signal processing unit 114.Together, the electronic components of the light source 108, signalreceiver 110, amplifier 112, and signal processing unit 114 form thedetection unit 116. A similar microbending optical fiber sensor isdescribed, for example, in U.S. Publ. No. 2012/0203117 by Chen, thedisclosure of which is herein incorporated by reference in its entirety.Chen discloses the simultaneous measurement of breathing rate and heartrate using a microbending multimode fiber optic sensor. A similarstructure was described by Chen et al. in “Portable fiber opticballistocardiogram sensor for home use,” 2012, vol. 8218, p.82180X-82180X-7, the disclosure of which is herein incorporated byreference in its entirety.

It was widely believed that a dual deformer structure, such as the onedescribed in Chen, was needed in order to produce a microbendingresponse large enough to make a body's micro-movements, indicative ofphysiological parameters such as heart beat and breathing, detectable.It was assumed and taught that a mesh structure needs to be placed oneach side of the optical fiber and aligned such that the fibers of onemesh layer are directly centered within the openings in the other meshlayer. Such a structure creates pressure points on both a top side and abottom side of the optical fiber, which facilitates deformation of thefiber.

Equations have been developed to explain this phenomenon and to identifythe ideal size and placement of the dual deformer structures. Such aprior art dual deformer structure is shown, for example, in FIGS. 2A and2B.

As illustrated in FIG. 2A and FIG. 2B, an optical fiber 250 is placedwithin a dual deformer structure 200 formed of a top layer of interwovenfibers 202 and a bottom layer of interwoven fibers 204. The opticalfiber 250 is sandwiched between the two layers of the dual deformerstructure 200. A cover 206 surrounds the dual deformer structure. Thetotal height of the dual deformer structure is l_(s). The intra-layerperiodicity of the deformer, meaning the distance between adjacent,parallel fibers in a mesh layer, is Λ_(D). The inter-layer periodicityof the deformer, which is the horizontal distance between fibers of theupper and lower deformer, is Λ_(s). In a dual deformer structure, anoutside applied force is concentrated on the optical fiber 250 at thelocations of the mesh fibers. In order for force to be applied evenlyonto the optical fiber 250, Λ_(s) must be half of Λ_(D). In order toachieve the greatest bending, and thus, the highest sensitivity, Λ_(s)must be half of Λ_(D).

When a lateral displacement misaligns the upper and lower mesh layers ofthe deformer 200, the applied force is no longer applied evenly onto thefiber 250 and Λ_(s) is no longer uniform. In this situation, as shown inFIG. 2B, Λ_(s) is replaced by Λ_(s1) (left side distance between fibersof upper mesh and lower mesh layers) and Λ_(s2) (right side distancebetween fibers of upper mesh and lower mesh layers).

For a graded index, micro-bending fiber, the optimum intra-layerperiodicity Λ_(D) of the deformer is obtained from the followingequation:

$\Lambda_{D} = \frac{2\; \pi \; {an}_{0}}{N.A.}$

as disclosed, for example, by Lagakos et al. in “Microbend fiber-opticsensor,” Appl. Opt., vol. 26, no. 11, pp. 2171-2180, June 1987, thedisclosure of which is herein incorporated by reference in its entirety.In the equation, a is the core radius, n₀ is the refractive index of thecore and N.A. is the numerical aperture of the fiber. Lagakos arrives atthe following values for the tested fiber: a=47.5 um, N.A.=0.13 andn₀=1.458 and concludes that an intra-layer periodicity (Λ_(D)) of 3.35mm achieves the highest microbending sensitivity for a pressure sensor.Chen uses the same equation in U.S. Publ. No. 2012/0203117, arriving ata periodicity of 1.68 mm.

In a dual deformer structure, the equation of intra-layer periodicityΛ_(D) is used to maximize the performance of a pressure sensor bymaximizing light loss under a given pressure force. The purpose is toachieve the highest fiber bending loss. In other words, to achieve thehighest ΔX.

k _(dual) ΔX=ΔF _(bending)

In a dual deformer structure,

$k_{dual}^{- 1} = \frac{\Lambda_{D}^{3}}{3E_{y}I_{bend}}$

Where E_(y) is Young's modulus, I_(bend) is the bending moment ofinertia, and Λ_(D) is the intra-layer periodicity. The bending moment ofinertia (I_(bend)) characterizes the stiffness of an elastic member, andfor an object with a circular cross-section, such as an optical fiber,I_(bend) is given by the equation:

$I_{bend} = \frac{\pi \; D_{fiber}^{4}}{64}$

Where D_(fiber) is the fiber diameter.

In dual deformer structures, the optical fiber 250, which is sandwichedbetween a pair of deformer mesh layers 202, 204, is constrained to bendin a regular pattern with periodicity Λ_(D). The deformer 200, inresponse to an appropriate environmental change ΔE, applies a force ΔFto the optical fiber 250 causing the optical fiber to deform by anamount ΔX. The transmission coefficient for light propagating throughthe bent fiber is in turn changed by an amount ΔT so that:

$\begin{matrix}{{\Delta \; T} = {\left( \frac{\Delta \; T}{\Delta \; X} \right)D\; \Delta \; E}} & (1) \\{where} & \; \\{{D\; \Delta \; E} = {\Delta \; X}} & (2)\end{matrix}$

Here D is a constant, which depends on the environment change ΔE. Interms of the applied force ΔF, equation (1) becomes

$\begin{matrix}{{\Delta \; T} = {\left( \frac{\Delta \; T}{\Delta \; X} \right)\; \Delta \; {F\left( {K_{f} + \frac{A_{s}Y_{s}}{l_{s}}} \right)}^{- 1}}} & (3)\end{matrix}$

Where K_(f) is the bend fiber force constant and A_(s)Y_(s)/l_(s) is aforce constant. Here A_(s), Y_(s) and l_(s) are: a cross sectional area,Young's modulus, and height of the dual deformer structure,respectively. The change in the photo detector output signal is thusused to detect the original environment perturbation ΔE.

Depending on the construction of the deformer, various environmentalparameters can, in principle, be sensed. The deformer converts thechange in the environmental parameter ΔE to a force ΔF on the bentfiber, according to the equation: ΔF=ΔE*C. For the generic dualdeformer, the parameter C can be expressed as a simple function ofdeformer parameters for the various environmental sensors. For apressure sensor, C is simply equal to the area of the deformer plateA_(p), thus equation (3) becomes

$\begin{matrix}{{\Delta \; T} = {{\frac{\Delta \; T}{\Delta \; X} \cdot {A_{p}\left( \; {K_{f} + \frac{A_{s}Y_{s}}{l_{s}}} \right)}^{- 1}}\Delta \; P}} & (4)\end{matrix}$

Where ΔP is the change in pressure. Thus a high sensitivity pressuresensor should have a constant A_(s)Y_(s)/l_(s) small enough that theeffective compliance is determined by the compliance of the opticalfiber, which is itself quite large. In this case, equation (4) becomes:

$\begin{matrix}{{\Delta \; T} = {{\frac{\Delta \; T}{\Delta \; X} \cdot A_{p}}\; K_{f}^{- 1}\Delta \; P}} & (5)\end{matrix}$

Equation (5) can be written in the form of equation (1), where ΔErepresents the environmental change, e.g., pressure and temperature, andD is a constant identified in the above equations for the variousenvironmental sensors. Under an environmental perturbation ΔE, the photodetector signal output i_(s) is given as

$\begin{matrix}{i_{s} = {\frac{{qeW}_{0}}{hv}\left( \frac{\Delta \; T}{\Delta \; X} \right)\; D\; \Delta \; E}} & (6)\end{matrix}$

where h is Planck's constant, v is the optical frequency, q is thedetector quantum efficiency, e is the electron charge, and W₀ is theinput optical power. Assuming a shot-noise-limited detection system, themean square photo-detector noise is given as

$\begin{matrix}{{i_{s}^{2}/i_{N}^{2}} = {\left( \frac{{qW}_{0}}{hv} \right)\left( \frac{\Delta \; T}{\Delta \; X} \right)^{2}\; {D^{2}\left( {\Delta \; E} \right)}^{2}\left( {2T\; \Delta \; F} \right)^{- 1}}} & (7)\end{matrix}$

The smallest signal that can be detected is given for the conditionS/N=1, which yields

$\begin{matrix}{{\Delta \; E_{\min}} = {{D^{- 1}\left( \frac{\Delta \; T}{\Delta \; X} \right)}^{- 1}\; \sqrt{\frac{2{Thv}\; \Delta \; f}{{qW}_{0}}}}} & (8)\end{matrix}$

The first factor is specific to the particular design of theenvironmental sensor; the second two factors, however, is general andapplies to all environmental microbending sensors. A genericmicrobending sensor can be defined as one which measures ΔE as definedby Eq. (2). Then combining Eqs. (2) and (8) yields

$\begin{matrix}{{\Delta \; X_{\min}} = {\left( \frac{\Delta \; T}{\Delta \; X} \right)^{- 1}\; \sqrt{\frac{2{Thv}\; \Delta \; f}{{qW}_{0}}}}} & (9)\end{matrix}$

which provides the minimum amount the optical fiber must bend in orderto detect an environmental perturbation. Based on these results, theminimum detectable environmental changes ΔE can be determined. Accordingto Chen in U.S. Publ. No. 2012/0203117, the optical fiber sensor withthe dual deformer structure was sensitive enough for heart beats andbreathing to be detectable environmental perturbations. Chen furthersuggests in “Portable fiber optic ballistocardiogram sensor for homeuse,” that such a structure may be sufficiently sensitive to detect BCGwaveforms. However, as mentioned above, the sensitivity of the sensorhaving a dual deformer structure is dependent on Λ_(s) being equal tohalf of Λ_(D). When Λ_(s) is not half of Λ_(D), the predesignedperiodicity of the microbend deformer Λ_(D) will not be the optimizedvalue necessary to achieve the best performance. Unless the upper andlower mesh layers are able to remain in proper alignment, the dualdeformer style optical fiber sensor of Chen lacks sufficient sensitivityand reliability to monitor BCG waveforms.

It may be possible to maintain proper alignment of the upper and lowerlayers of mesh with the addition of specialized support structures;however, doing so greatly increases the cost and complexity of thesystem, which makes the system impractical for consumer applicationssuch as long-term vital sign monitoring systems.

Without the costly support structures, the dual deformer structure isnot suitable for detecting BCG waveforms in practice outside of alaboratory. While the two mesh layers of the dual deformer may begin inan aligned configuration and generate sensitive and reliable results ina laboratory, when used by consumers, the slipping of one deformer layerrelative to the other deformer layer often results. In real applicationsof vital sign monitoring, where an individual is sitting or lying on anoptical fiber sensor, there are pressures and forces of differentdirections routinely applied on the sensor. This pressure or force canbe divided into vertical and horizontal components. The verticalcomponent is the component that results in a downward force on theoptical fiber, and accordingly, microbending of the fiber. However,shifts in body weight will almost always have a horizontal component aswell. The horizontal component caused by a patient moving induces aforce that may change the periodicity between the mesh layers. Thechange in Λ_(s) and Λ_(D) significantly reduces the sensitivity of thesystem since the greatest amount of microbending will no longer beachievable. Moreover, the change in Λ_(s) and Λ_(D) significantlyreduces the reliability of the system since the placement of the forceson the optical fiber become unpredictable all calculations performed bythe signal processor in the system are based on the assumption thatΛ_(s) and Λ_(D) have remained constant. Thus, as the sensor system isused, the dual deformer style optical fiber sensors become unreliableand sensitivity decreases. This is not acceptable for commercialapplications. Accordingly, a different approach and different structureis needed.

Single Deformer Sensor

As stated above, it has previously been believed that a dual deformerstructure was necessary to achieve an amount of microbending sufficientto detect vital signs. It is the present author's surprising discoverythat an amount of microbending sufficient to detect vital signs,including BCG waveforms, can be achieved using a single deformerstructure, if the optical fiber sensor utilizes includes an opticalfiber having specific properties. As described further herein, aspecific relationship has been discovered between characteristics of theoptical fiber and the single mesh layer that make such a systemconducive to reliable, sensitive results.

FIG. 3 illustrates one embodiment of the sensor structure in accordancewith the present disclosure. The sensor 300 includes a multimode opticalfiber 302 and a single layer of mesh 304, which are together heldbetween a front cover 306 and a back cover 308 to form a sensor sheet.One end of the optical fiber 302 is connected to a light source 310,which in a preferred embodiment, is an LED light source operated by anLED driver 312. Another end of the optical fiber 302 is connected to anoptical signal receiver 314. An amplifier 316 is coupled to the opticalsignal receiver 314 to amplify the optical signal large enough forprocessing by an analog-to-digital converter 318. The optical andelectrical components (310, 312, 314, 316, 318) are connected to acontrol and processing module 320. The provided sensor is configured foroptical intensity monitoring. Input light is generated from the lightsource 310 and transmitted to optical multimode fiber 302. In thepresence of an external force generated by body weight, heartbeat,respiration, or other physiological parameter, this force is uniformlydistributed on the fiber 302 and mesh structure 304. These forcesmicrobend the multimode fiber, and some light leaks out due to themicrobending effect. Optical signal receiver 314 receives the residuallight and changes in the amount of light intensity are processed anddetermined by the control and processing module 320.

Unlike sensors of the prior art which use two microbending deformerlayers, the sensor of the present disclosure uses a deformer structureformed of a single layer of mesh.

FIGS. 4A and 4B illustrate one embodiment of the deformer structureaccording to the present disclosure. In the present deformer structure,one single layer of mesh 402 is used to achieve micro-bending on amultimode fiber 401. As used herein, mesh may refer to any suitablematerial having a repeating pattern of through holes. In someembodiments, the mesh is formed of interwoven fibers, such as, forexample, polymeric fabric fibers, natural fabric fibers, compositefabric fibers, metallic fibers, or other fibers. In this mesh layer,a_(o) is the open area of the through holes 403. The diameter or widthof each mesh fiber in the x direction is d₁. The diameter or height ofeach mesh fiber in the z direction is d₂. The width of each mesh openingis w. When an outside force caused by body weight, a heartbeat,respiration, or other physiological parameter is applied on the sensor,this force is uniformly distributed onto an upper cover 404 and themultimode optical fiber 401.

In the new sensor structure of the present disclosure, a mesh structure402, such as a polymeric open mesh fabric, is used as the single layerdeformer, and a cover 404, such as silicone material, is configured tosurround the multimode optical fiber 401 and the mesh layer 402 todistribute uniformly any force applied on the sensor. The outer cover404 encloses and is bonded to the optical fiber 401 and deformerstructure 402.

In the one layer deformer structure of the disclosure, an appliedoutside force is uniformly distributed along the optical fiber length.The force per unit length is denoted by F_(dist), and the amount ofdeformation is:

k_(distributed)  Δ X = Δ F_(dist) where$k_{distributed}^{- 1} = \frac{\Lambda_{L}^{4}}{8\; E_{y}I_{bend}}$

and where E_(y) is Young's modulus, I_(bend) is the bending moment ofinertia, and Λ_(D) is the length of period. The bending moment ofinertia characterizes the stiffness of an elastic member, and for anobject with a circular cross section, such as an optical fiber, it isgiven by:

$I_{bend} = \frac{\pi \; D_{fiber}^{4}}{64}$

where D_(fiber) is the fiber diameter. So,

$k_{distributed}^{- 1} = \frac{8\Lambda_{L}^{4}}{E_{y}\pi \; D_{fiber}^{4}}$

In sensors utilizing single layer deformers,

Λ_(L) =d ₁ +w

Combining this information together yields the amount the optical fiberwill bend in response to an application of force in a structure having asingle layer deformer structure. In particular:

${\Delta \; X} = {\left( {\Delta \; F_{dist}} \right)\frac{8\left( {d_{1} + w} \right)^{4}}{E_{y}\pi \; D_{fiber}^{4}}}$

As shown, a sensor utilizing a deformer structure made of only one layerof mesh has different bending parameters than a dual deformer structure.In the currently provided embodiments, each of which employ a singlelayer deformer structure, the amount of bending depends not only on theapplied force but also the diameter of the optical fiber, the diameterof the mesh fiber, and the size of the openings in the mesh. Bybalancing these parameters, a sensor can be created that bends asufficient amount to detect light loss in response to the force of amicro-movement of a body on the sensor.

Relationship Between Mesh Layer and Optical Fiber Parameters

It is the unexpected discovery of the present authors that a morereliable and sufficiently sensitive sensor can be created using thedeformer structure described above, if the size of the deformerstructure is appropriately selected and paired with an appropriatelysized optical fiber. In particular, it was found that eliminating thesecond mesh layer significantly reduces the noise and error that resultsfrom the two mesh layers sliding relative to each other. Moreover, itwas found that an amount of microbending sufficient to achievedetectable light loss can be obtained when the deformer only includesone mesh layer, if a specialized multimode optical fiber is used. It isboth the characteristics of the optical fiber and their relationship tocharacteristics of the mesh layer that result in sufficientmicrobending.

For example, the specialized multimode optical fiber of the presentdisclosure has a large core diameter configured to receive and transmita relatively large amount of light. Moreover, the specialized multimodeoptical fiber is a highly flexible bare fiber, which includes theoptical fiber core, outer cladding, and a coating layer, but does notinclude a tight buffer layer. Rather than including a tight bufferlayer, additional protection to the optical fiber and the system isprovided by the highly flexible cover 404, which surrounds both thefiber and the deformer structure.

It is a discovery of the present authors that a bare multimode opticalfiber will bend a sufficient amount in response to physiologicalparameters of interest if an appropriate ratio of core to cladding isselected, if the mesh layer of the deformer is an appropriate size forthe optical fiber, and if an appropriate power level is used.

The optical fiber sensors of the various embodiments disclosed hereineach have a novel configuration that enables sufficient microbending tobe achieved using a deformer having only one mesh layer. Each of theembodiments described below is configured to microbend a sufficientamount to monitor BCG waveforms, heart rate, breathing rate, and otherphysiological parameters.

A multimode optical fiber of the current disclosure is provided in FIG.5. As shown, the optical fiber 500 is formed of an inner core fiber 502,a cladding layer 504, and an outer coating 506. In various embodiments,the inner core 502 has a diameter equal to or greater than 50% of thediameter of the cladding layer 504. In preferred embodiments, thediameter of the inner core 502 is greater than 50% of the diameter ofthe cladding layer 504. In some embodiments, the diameter of the innercore 502 is substantially greater than 50%, for example, at least 75%,80%, or 90% of the diameter of the cladding layer 504. As used herein,the diameter of the inner core refers to the diameter across the innercore; the diameter of the cladding layer refers to the diameter acrossthe entirety of the cladding layer and the inner core; and the diameterof the outer coating or the total diameter of the optical fiber refersto the diameter across the entirety of the outer coating, the claddinglayer, and the inner core. In some embodiments, the diameter of thecladding layer is 125 μm. In some such embodiments, the inner core has adiameter of at least 62.5 μm. In some embodiments, the inner core has adiameter greater than 62.5 μm. For example, in some such embodiments,the inner core has a diameter of at least 80 μm, at least 90 μm, atleast 100 μm, or at least 110 μm. In one embodiment, the optical fiberhas an inner core diameter of 100 μm and a cladding layer diameter of125 μm. In various embodiments, an outer coating adds additional widthto the optical fiber. In some such embodiments, the total diameter ofthe optical fiber is 250 μm. In another set of embodiments, the opticalfiber has a cladding layer diameter of 250 μm. In some such embodiments,the inner core has a diameter equal to, or greater than, 125 μm. In somesuch embodiments, the inner core has a diameter greater than 150 μm,greater than 160 μm, greater than 170 μm, greater than 180 μm, greaterthan 190 μm, greater than 200 μm, greater than 210 μm, or greater than225 μm.

In various embodiments of the disclosed optical fiber sensor, the meshlayer of the deformer is configured such that the open area betweenfibers a_(o) is between 30% and 60% of the total mesh surface area. Insome embodiments, the through-holes of the mesh layer are sized toreceive an entire diameter of the optical fiber. In some embodiments,the through-holes of the mesh layer are sized to receive the width of anoptical fiber structure, including the optical fiber and surroundingouter coating. Thus, in some embodiments, the opening of the mesh layeris 100% to 300% if the total diameter of the optical fiber. In someembodiments, the opening of the mesh layer is 130% to 170% of the totaldiameter of the optical fiber.

In still another embodiment, the mesh opening w is preferably selectedto be between 200 and 750 μm. In an alternative embodiment, the meshopening is up to three times greater than the total optical fiberdiameter.

In some embodiments, a diameter of each mesh fiber is within 25% of thetotal diameter of the optical fiber. In some embodiments, each meshfiber has a diameter equal to 75% to 110% of the total diameter of theoptical fiber. In some embodiments, each mesh fiber has a diametergreater than 70% and less than 100% of the total diameter of the opticalfiber. In some embodiments in which the optical fiber has a claddinglayer diameter of 125 μm and a total diameter of 250 um, a diameter ofeach mesh fiber is selected to be in the range of 180 to 240 μm.

In a preferred embodiment, the multimode optical fiber has a numericalaperture less than or equal to 0.29, a 100 μm core diameter, a 125 μmcladding layer diameter, and a 250 μm total diameter. In someembodiments, for example, in this preferred embodiment, the mesh fiberhas a diameter between 180 and 240 μm. In some embodiments, for example,in this preferred embodiment, the mesh opening w is sized between 330and 375 μm.

The optical fiber may be any length suitable for the area of detection.In some embodiments, the sensor includes at least 10 meters of multimodeoptical fiber. In some such embodiments, the multimode optical fiber isarranged along a plane and wound, coiled, or snaked along the plane.Macrobending effects can significantly decrease the microbending effectand are preferably avoided. Macrobending losses are high in 0.29numerical aperture fiber for bends of less than 1.5 cm in diameter.Thus, in some embodiments, a bending diameter greater than 1.5 cm isused when laying the optical fiber on the plane. In some embodiments,the optical fiber is arranged directly on the mesh layer.

Various embodiments of the present disclosure are directed to a sensorconfigured to achieve high sensitivity and reliable performance at lowmanufacturing cost and complexity.

Fiber sensor leads also affect the sensitivity. In some embodiments, alead fiber is needed to couple the light from the light source to thefiber, and another lead fiber is needed to connect to the detector. Insuch embodiments, the core sizes of the lead and sensing fibers shouldbe approximately the same to minimize fusion loss. In preferredembodiments of the disclosure, the need for separate fiber sensor leadsis eliminated by directly connecting the multimode fiber to theelectronic and optical components.

In a preferred embodiment, a low power, low cost 850 nm or 1310 nmcentral wavelength LED with 165 nm Full width at half maximum (FWHM) isused as light source. A photo detector range (770 nm to 860 nm or 1100nm to 1650 nm) photo detector with 0.4 A/W responsivity is used as theoptical receiver. In other embodiments, any suitable LED light source orother low power, low cost light source may be used. Additionally, inother embodiments, any suitable light receiver may be used.

Electro-Optic Unit and Controller

FIG. 6 illustrates a block diagram of an optical intensity microbendingfiber sensor system 600 for detecting and displaying physiologicalparameters of a body. In the illustrated embodiment, a low cost fibersensor sheet 601 is provided, which may include any embodiment of atopcover, multimode optical fiber, mesh layer, and back cover describedelsewhere herein, for example, in relation to FIGS. 3-5. A light source602 supplies optical radiation to the multimode fiber embedded withinsensor sheet 601. The light source 602 may be a broadband optical source165 nm, FWHM LED light source 1310 nm, or any other suitable lightsource. An LED driver 604 drives the optical light source 602 and iscontrolled by a processor 606 executing instructions stored in memory608. The other end of the multimode fiber is connected to the opticalsignal receiver 610, which is comprised of a photo detector. In someembodiments, the photo detector has a detection range from 1100 nm to1650 nm and 0.4 A/W responsivity. The optical signal receiver 610converts the optical intensity into an analog electrical signal, whichis then amplified by an electrical amplifier 612. An analog-to-digitalconverter 614 converts the analog electrical signal into a digitalsignal that is transmitted to and processed by the processor 606. Insome embodiments, a user interface 616 may be provided and used by auser to control some or all of the device's functionality. The digitalsignal is optionally processed by the processor 606, and the raw orprocessed digital signal is transmitted to a remote system 650 todisplay and further process the signal. This remote system 650 can be asmart phone, tablet, other mobile computing device, or other computerwith appropriate communication capabilities. In some embodiments, theraw or processed digital signal is transmitted to the remote system 650wirelessly, for example, via radiofrequency (RF) signals utilizing atransmitter 620. The transmitter 620 of some embodiments is configuredto transmit Wi-Fi®, Bluetooth®, or other RF signals. In someembodiments, the remote system 650 can in turn control and activate thesensor system 600 by sending signals to a receiver 618, which may be,for example, an RF signal receiver.

The processor 606, the memory 608, and the signal processing components(e.g., the amplifier 612 and the analog-to-digital converter 614) mayinclude a combination of hardware and software, which is configured tocontrol the frequency, intensity, and/or activation of the light emittedby the light source, and which is further configured to convert thesignals received from the signal receiver into meaningful data. Oneskilled in the art will appreciate that many different structuralcomponents and architectures may be used to achieve such functionality.Although illustrated separately, it is to be appreciated that thevarious blocks of the system need not be separate structural elements.For example, in the processor in data communication with the memory maybe embodied in a single chip or two or more chips.

The processor 606 may be a general purpose microprocessor,microcontroller, a digital signal processor (DSP), a field programmablegate array (FPGA), an application specific integrated circuit (ASIC), orother programmable logic device, or other discrete computer-executablecomponents designed to perform the functions described herein. Theprocessor may also be formed of a combination of computing devices, forexample, a DSP and a microprocessor, a plurality of microprocessors, oneor more microprocessors in conjunction with a DSP core, or any othersuitable configuration.

In various embodiments, the processor 606 is coupled, via one or morebuses, to the memory 608 in order to read information from and writeinformation to the memory 608. The processor 606 may additionally oralternatively contain memory 608. The memory 608 can include, forexample, processor cache. The memory 608 may be any suitablecomputer-readable medium that stores computer-readable instructions forexecution by computer-executable components. For example, thecomputer-readable instructions may be stored on one or a combination ofRAM, ROM, flash memory, EEPROM, hard disk drive, solid state drive, orany other suitable device. In various embodiments, the computer-readableinstructions include software stored in a non-transitory format. Theprocessor 606, in conjunction with the software stored in the memory608, executes an operating system and stored software applications.Various methods described elsewhere herein may be programmed as softwareinstructions stored in the memory 608.

The user interface 616 may include a user input device, such as abutton, a toggle, a switch, a touch screen, or a keypad, and/or anoutput device such as a display screen, light display, audio output, orhaptic output. The user input device may be configured to receive usercommands to power the sensor on and off. In some embodiments, data abouta user may also be input via the user input device.

The receiver 618 of various embodiments receives and demodulates datareceived over a communication network. The transmitter 620 prepares dataaccording to one or more network standards and transmits data over acommunication network. In some embodiments, a transceiver antenna actsas both a receiver and a transmitter. Additionally or alternatively, insome embodiments, the system includes a databus for sending and/orreceiving data to one or more remote components via a wired connection.

In some embodiments, the processor 606 is configured to compute appliedforces from changes in propagated light intensity. In some embodiments,the processor 606 is configured to compute one or more vital signs of auser from the data on applied forces. In some embodiments, some or allsuch data is transmitted via a wired or wireless connection to theremote system 650 for storage and/or display.

In some embodiments, the processor 606 is configured to extract aheartbeat waveform, respiration waveform, and movement waveform from theraw signal. One example of such signal extraction is shown in FIGS.7A-7D. The raw signal 700 a is received at the signal processingcomponents. As illustrated in FIG. 7A, in time window 702, the lightintensity is at the highest level, which means there is no user on thesensors. There is no light loss due to the bending of the fiber. Timewindow 704 represents the moment when a user sits, stands, or lays onthe sensor, which makes the received light intensity drop significantlyin a very short period; the signal then maintains a consistently lowlevel as in 706. At illustrated time windows 706 and 710, the user issitting or lying on the sensor without moving. During such times, thelight intensity is kept within a relatively narrow range. By amplifyingthe signal in time windows 706 or 710, a detailed signal is detectable,which includes a heartbeat waveform and respiration waveform, as shownin FIG. 7B as signal 700 b. When a user remains sitting, standing, orlying on the sensor but shifts positions or moves, a signal such as thesignal shown in time window 708 results. As shown, during such times,the optical intensity changes in a range larger than the intensitychange caused by respiration and heartbeats but in a range much smallerthan the intensity change caused by complete addition or removal of thebody from the sensor. The processor 606 of some embodiments isconfigured to identify the movement frequency and amplitude from thissignal. The time window 712 depicts the signal as the user leaves thesensor. The received light intensity then recovers to the originallevel, as shown in 714.

To extract the heartbeat waveform, also called a ballistocardiography(BCG) waveform, from such a signal, the processor 606 may combinedifferent stage high pass and low pass filters to remove noise. The BCGwaveform 700 c, shown for example in FIG. 7C, provides detailedinformation for every heartbeat. The BCG waveform acquired by thepresent system has very clear H, I, J, K, L, and M peaks. By identifyinga heartbeat peak value, time differences between adjacent heartbeats720, 722 can be computed. The processor 606 may calculate heart rate bygathering all the time differences and transferring them into afrequency in time domain. The processor 606 may calculate heart ratevariability by calculating the average time difference between adjacentpeaks over a certain time period. The processor 606 may determine astress level, such as a mental stress level, from the heart ratevariability.

The respiration signal 700 d of FIG. 7D is easier to extract thanheartbeat signals, because the light loss caused by respirationbiomechanical movement is much larger than the light loss caused byheartbeat movements. In some embodiments, the heartbeat signal is notfiltered out from the respiration waveform. In some embodiments, arespiration rate calculation is performed by the processor 606 byidentifying a peak value of each breathing waveform to get a timedifference 730 between two respiration cycles.

Methods of Use

FIG. 8 illustrates one embodiment of a signal filtering and dataanalysis process 800 performed by a processor of the presently describedoptical fiber sensor monitoring system. Signal processing can beperformed to extract the heartbeat/respiration waveforms from the rawsignal when a user is on the sensor and not moving a lot. Such a processmay be performed, for example, by the processor 606 of FIG. 6. In someembodiments, the processor receives raw digital data from ananalog-to-digital converter, as shown at block 802. When a digitalsignal is received, the optical intensity absolute value may be checked,as shown at block 804. When an optical intensity absolute value isknown, the processor of some embodiments compares the received signal tothe optical intensity absolute value to determine whether a user is onthe sensor, as shown at block 806. If a user is on the sensor, there issignificant light loss and a resultant decline in optical intensity.When a user is not on the sensor, the hardware of some embodimentsoperates in sleep mode to save battery power, as shown at block 808.When high light loss is detected, the processor of some embodimentsproceeds to determine whether the user is moving, as shown at block 810.If the processor detects repeated or continual relatively large lightloss changes, the processor defines this moment as a moving moment andoutputs a movement waveform, as shown at block 812. If the lightintensity remains relatively constant with little changes, this momentmay be defined as a heartbeat/respiration processing moment and the rawdigital data signal is split into two channels, as shown at block 814.

In the illustrated embodiment, at block 816, the signal in one channelundergoes multiple stages of filtering. In one embodiment, five stagesof high and low band pass filtering are performed to remove the noise inthe raw data and isolate the heartbeat waveform, as shown at block 818.There are many methods that may be used to process the heartbeatwaveform. In various embodiments, a method is used that is relativelysimple and requires low processing power, so as to be suitable forperformance by a portable sensor having relatively low processing andbattery capabilities. In some embodiments, a combination of time domainand frequency domain analysis is used to get a robust heartbeat value.In such embodiments, the heartbeat waveform is split into two channels.One channel is for time domain analysis, while another channel is forfrequency domain analysis. In time domain analysis, a normalizationprocess 820 is performed to make data consistent for ease of analysis.Data may be kept in one fixed window by zooming in and zooming out, asshown at block 822. In frequency domain, data is squared, as shown atblock 824, and a Fast Fourier Transformation is performed at 826 to getthe peak frequency. The peak frequency is the heartbeat rate. Heartbeatrate matching is done at block 828 to reduce error rate. In variousembodiments, the processor outputs a heartbeat rate value at 830.

In some embodiments, another channel of the raw digital signal isprocessed at block 832 by averaging the signal and applying a low passfilter. A breathing waveform is output at block 834. In someembodiments, a time window of multiple data points, for example, 1000data points, is saved at block 836 to find the peak value in the timedomain at block 838. By identifying the peak value of the respirationwaveform, the respiration rate can be obtained, and average respirationrate may be matched to the latest respiration rate at block 840 to get abreathing rate. As shown at block 842, in some embodiments, theprocessor outputs the breathing rate.

The optical fiber sensors and associated methods of signal filtering anddata analysis described above may be used to determine one or morephysiological parameters of a patient. One example of a method ofdetermining physiological parameters of a patient is shown in FIGS. 9Aand 9B. As shown at block 910, in various embodiments a sensor ispositioned under the body of a patient. The patient may be positioned tostand, sit, or lay on the sensor. The sensor may be any optical sensorembodiment described in the present disclosure. At block 920, the systemdetects a change in an intensity of light traveling through an opticalfiber of the sensor. In various embodiments, the change in lightintensity is detected at a light receiver and processed by a processor.As described above, the change in light intensity corresponds to fiberdeformation caused by a movement of the body. The movement of the bodymay be a micro-movement (such as breathing or a heartbeat) or amacro-movement (such as a shift in body position). Using the methods ofFIG. 8 or other methods known to those skilled in the art, the processorof the provided optical fiber sensor determines a physiologicalparameter from the change in light intensity, as shown at block 930. Thephysiological parameter may be, for example, a BCG waveform, a heartrate, a breathing rate, or other vital sign or parameter of interest.

In some embodiments, determining the physiological parameter includesdetermining beat-to-beat blood pressure changes. As shown in FIG. 9B,such a determination may be performed by identifying a BCG waveform, asshown at block 932 and described in detail above, receiving an EEGwaveform, as shown at block 934, and calculating a time between an Rpeak of the EEG waveform and a J peak of the BCG waveform, as shown atblock 936. The EEG waveform may be received by a processor from an EEGsensor. In some embodiments, the EEG sensor is external to the opticalfiber sensor described herein. The time between the R peak and the Jpeak is indicative of the beat-to-beat pressure change.

In some embodiments, methods performed by a healthcare professionalinclude placing a fiber optic sensor, such as any of the single deformerstructure sensors described herein, under a patient. The sensor may bedisposed, for example, within a seat cushion, chair, bed, mattress,mattress pad, rug, mat, or any other suitable structure on which apatient can sit, lay, or stand. The method performed by the healthcareprofessional may further include activating the sensor and viewingphysiological parameters output by the sensor. In some embodiments, thephysiological parameter includes a BCG waveform. The method of someembodiments further includes diagnosing a health condition, at least inpart, from abnormalities detected in the BCG waveform.

In various embodiments disclosed herein, the optical fiber sensor isconfigured to acquire very clear, reliable, and reproducible BCGwaveforms. While some semblance of a BCG waveform may be detectable bysome existing optical fiber sensors having a dual deformer structure,the waveforms are insufficient for clinical monitoring and diagnosticpurposes. For example, the BCG waveforms acquired by prior art systemsare often unreliable due to noise and inaccuracies that result when thelayers of the deformer become misaligned. Moreover, the BCG waveformsacquired by prior art systems are unstable. A clear, stable BCG waveformincludes several characteristic features including featuresconventionally denoted as H, I, J, K, L, M, and N. As shown in FIG. 10A,a BCG waveform acquired by one prior art system generated a veryunstable BCG waveform in which the H wave is frequently missing. Incontrast, one embodiment of the system of the current disclosureproduced the very clear, stable, reliable, and reproducible BCG waveformof FIG. 10B.

In some methods of use, such a reliable BCG waveform may be relied on bya healthcare professional to facilitate diagnosis of one or more healthconditions. For example, identification of a characteristic abnormalityin a BCG waveform may be used to help diagnose angina pectoris,asymptomatic coronary artery disease, acute myocardial infarction,hypertension, coarctation of the aorta, mitral stenos is, and othercardiac conditions, as described, for example, in E. Pinheiro et al.,“Theory and Developments in an Unobtrusive Cardiovascular SystemRepresentation: Ballistocardiography,” The Open Biomedical EngineeringJournal, 2010, 4, pp. 201-216, the contents of which is hereinincorporated by reference in its entirety.

CONCLUSION

As used in the description and claims, the singular form “a”, “an” and“the” include both singular and plural references unless the contextclearly dictates otherwise. For example, the term “a sensor” mayinclude, and is contemplated to include, a plurality of sensors. Attimes, the claims and disclosure may include terms such as “aplurality,” “one or more,” or “at least one;” however, the absence ofsuch terms is not intended to mean, and should not be interpreted tomean, that a plurality is not conceived.

The term “about” or “approximately,” when used before a numericaldesignation or range (e.g., to define a length or pressure), indicatesapproximations which may vary by (+) or (−) 5%, 1% or 0.1%. Allnumerical ranges provided herein are inclusive of the stated start andend numbers. The term “substantially” indicates mostly (i.e., greaterthan 50%) or essentially all of a device, substance, or composition.

As used herein, the term “comprising” or “comprises” is intended to meanthat the devices, systems, and methods include the recited elements, andmay additionally include any other elements. “Consisting essentially of”shall mean that the devices, systems, and methods include the recitedelements and exclude other elements of essential significance to thecombination for the stated purpose. Thus, a system or method consistingessentially of the elements as defined herein would not exclude othermaterials, features, or steps that do not materially affect the basicand novel characteristic(s) of the claimed invention. “Consisting of”shall mean that the devices, systems, and methods include the recitedelements and exclude anything more than a trivial or inconsequentialelement or step. Embodiments defined by each of these transitional termsare within the scope of this disclosure.

While embodiments described herein include the terms “patients,”“person,” and/or “individual” for simplicity of description, it will beappreciated by one skilled in the art that various embodiments describedherein are applicable to, and contemplated to be applied to, monitoringof vital signs in any mammal, including pets, livestock, and healthyindividuals such as office workers, babies, or others who are notpatients in a healthcare setting.

The examples and illustrations included herein show, by way ofillustration and not of limitation, specific embodiments in which thesubject matter may be practiced. Other embodiments may be utilized andderived therefrom, such that structural and logical substitutions andchanges may be made without departing from the scope of this disclosure.Such embodiments of the inventive subject matter may be referred toherein individually or collectively by the term “invention” merely forconvenience and without intending to voluntarily limit the scope of thisapplication to any single invention or inventive concept, if more thanone is disclosed. Thus, although specific embodiments have beenillustrated and described herein, any arrangement calculated to achievethe same purpose may be substituted for the specific embodiments shown.This disclosure is intended to cover any and all adaptations orvariations of various embodiments. Combinations of the aboveembodiments, and other embodiments not specifically described herein,will be apparent to those of skill in the art upon reviewing the abovedescription.

What is claimed is:
 1. A method of detecting a physiological parameter,the method comprising: positioning a sensor under a body, the sensorcomprising: a multi-mode optical fiber comprising an inner core, acladding layer, and an outer coating, wherein a core diameter is greaterthan 50% of a cladding diameter; an LED (light emitting diode) lightsource coupled to a first end of the optical fiber; an LED driverelectrically coupled to the LED light source and configured to regulatea power level of the LED light source; a receiver coupled to a secondend of the optical fiber; and a deformer structure consisting of asingle mesh layer formed of mesh having openings disposed therein, theopenings having a surface area between 30% and 60% of a total surfacearea of the mesh layer; wherein the optical fiber is arranged in a planein contact with a surface of the deformer structure, and the deformerstructure is configured with an amount of microbending sufficient todetect the physiological parameter; detecting, by the receiver, a changein an intensity of light traveling through the optical fiber, whereinthe change in light intensity corresponds to fiber deformation caused bya movement of the body; and determining a physiological parameter fromthe change in light intensity.
 2. The method of claim 1, whereindetermining the physiological parameter comprises determining aballistocardiography (BCG) waveform of the body.
 3. The method of claim1, wherein determining the physiological parameter further comprises:determining an EEG waveform of the body; and calculating a time betweenan R peak of the EEG waveform and a J peak of the BCG waveform todetermine beat-to-beat blood pressure changes.
 4. The method of claim 1,wherein determining the physiological parameter comprises: recording thesignal detected at the receiver; converting the signal to a digitalwaveform; filtering out breathing and body movement waveforms from thedigital waveform to extract a heartbeat waveform; identifying heartbeatpeak values from the heartbeat waveform by separating the heartbeatwaveform into a first channel for time domain analysis and into a seconddomain for frequency domain analysis; and applying a Fast FourierTransform in the frequency domain to obtain the heartbeat rate value. 5.The method of claim 1, wherein the mesh layer is formed of interwovenfibers; a flexible cover is configured to surround both the opticalfiber and the deformer structure to form both an upper cover on theoptical fiber and a back cover under the deformer structure such as todistribute uniformly any force applied on the sensor; the optical fiberis arranged directly on the mesh layer.
 6. The method of claim 5,wherein the sensor has the multimode optical fiber and the single layerof mesh together held between the upper cover and the back cover to forma sensor sheet; the sensor sheet is configured that a first portion ofthe optical fiber is capable of microbending into an opening of thesingle mesh layer and a second portion of the optical fiber is capableof flexing against the mesh under an applied outside force onto thesensor.
 7. The method of claim 1, wherein an amount the optical fiberwill bend ΔX in response to the application of force per unit length ΔFon the sensor is as defined by Eq. (1): $\begin{matrix}{{\Delta \; X} = {\left( {\Delta \; F_{dist}} \right)\frac{8\left( {d_{1} + w} \right)^{4}}{E_{y}\pi \; D_{fiber}^{4}}}} & (1)\end{matrix}$ where d1 is a diameter or width of each mesh fiber in thex direction, w is a width of a mesh opening wide, Ey is Young's modulus,and D_(fiber) is a diameter of the optical fiber.
 8. The method of claim1, wherein detecting by the receiver further comprising steps of:generating input light from the LED light source and transmitting tooptical multimode fiber; detecting raw data of the intensity of lightand converting the raw data into an analog electrical signal by thereceiver; amplifying the analog electrical signal by an electricalamplifier; converting the analog electrical signal into a digital signalby an analog-to-digital converter; and transmitting the raw data of theintensity of light in a form of digital signal after theanalog-to-digital converter to a processor.
 9. The method of claim 8,wherein determining a physiological parameter comprises: computingapplied forces from change in an intensity of light via the processor;and/or, computing one or more vital signs of a user from the data on theapplied forces via the processor; and/or extracting a heartbeatwaveform, respiration waveform, or movement waveform from the raw signalvia the processor; and/or identifying a movement frequency and amplitudefrom the signal via the processor.
 10. The method of claim 8, whereinthe detected intensity of light is at the highest level when there is nouser on the sensors; the detected intensity of light drops significantlyin a very short period at a moment when a user sits, stands, or lays onthe sensor; the detected intensity of light keeps within a relativelynarrow range when the user is sitting or lying on the sensor withoutmoving; the detected intensity of light recovers to the original levelwhen the user leaves the sensor.
 11. The method of claim 8, afterreceiving the raw data from the analog-to-digital converter, furthercomprising: checking an optical intensity absolute value by theprocessor; and comparing the received signal to the optical intensityabsolute value to determine whether a user is on the sensor.
 12. Themethod of claim 11, if a user is not on the sensor, further comprising:operating in sleep mode to save battery power.
 13. The method of claim11, if there is a significant light loss and a resultant decline in theintensity of light, further comprising: determining whether the user ismoving by the processor.
 14. The method of claim 13, if a repeated orcontinual relatively large light loss change is detected, furthercomprising: defining this moment as a moving moment and outputs amovement waveform.
 15. The method of claim 13, if the intensity of lightremains relatively constant with little changes, further comprising:splitting the raw digital data signal into two channels.
 16. The methodof claim 15, wherein the signal in one channel undergoes multiple stagesof filtering, which further comprises: combining different stage highpass and low pass filters to remove noise from the signal, and isolatingheartbeat waveform; splitting the heartbeat waveform into two channelsincluding one channel for time domain analysis while another channel forfrequency domain analysis.
 17. The method of claim 16, wherein multiplestages of filtering further comprising: performing a normalizationprocess in time domain analysis to make data consistent for ease ofanalysis, and keeping data in one fixed window by zooming in and zoomingout; and squaring the data in frequency domain, and performing a FastFourier Transformation to get heartbeat rate value; matching heartbeatrate value from time and frequency domain to reduce error rate; andoutputting heartbeat rate value by the processor.
 18. The method ofclaim 15, wherein another channel of the raw digital signal is furtherprocessed, which comprises: averaging the signal and applying a low passfilter; and outputting a breathing waveform.
 19. The method of claim 18,further comprising: saving a time window of multiple data points;finding a peak value a the time domain; identifying the peak value of arespiration waveform; and obtaining the respiration rate.
 20. The methodof claim 19, further comprising: matching an average respiration rate tothe latest respiration rate; and outputting the breathing rate by theprocessor.